Frequency modulated-amplitude modulated receiver



June 7,1949, w R, KOCH 2,472,301

FREQUENCY MODULATED-AMPLITUDE MODULATED REGEIVERS w. R. KocH I 2,472,301

FREQUENCY MODULATED-AMPLITUDE MODULA'IED RECEIVERS June 7, 1949.

2 Sheens-SheerI 2 Original Filed Feb. 5, 1945 SLOW ,4.1%6.

ELAV D/ODE R Em Am 0 rm a ai H mm F v n c M 3 .C nu L S r f 6 AAAAAAAAAA.rc s AV FA INVENToR. M//NF/ELD Q KOCH Patented June 7, 1949 UNITEDSTATES PATENT OFFICE FREQUENCY MODULATED-AMPLITUDE MODULATED RECEIVERWinfield R. Koch, Haddonleld, N. J., assignor to Radio Corporation ofAmerica, .a corporation of Delaware (Cl. Z50- 20) 4 Claims.

My present invention relates to receivers of frequency modulated (FM) oramplitude modulated (AM) carrier waves, and more particularly to noveland improved receivers of FM or AM carrier waves, This application is adivision of my application Serial No. 521,193, led February 5 1944, nowPatent No. 2,429,726.

In his U. S.. Patent No. 2,296,092, granted September 15, 1942, M. G.Crosby has disclosed a differential detector circuit adapted to receiveFM and/or AM carrier waves. In his detector circuit Crosby providesautomatic volume control (AVC) voltageat the detector output in responseto either FM or AM reception. The AVC voltage is derived from theseparate rectified voltages of the balanced rectiiiers added in aidingphase, while the modulation signal corresponding to the -frequencymodulation of a carrier wave is derived separate and independent FM andAM signal' channels to the detector and yet lbe able to provide the sameoutput voltages as in the Crosby system. When receiving FM signals witha receiver having a substantially flat-topped selec.- tivelycharacteristic and no amplitude limiter used, it is desirable to provideAVC voltage from the opposed diodes of the discriminator-detectorcircuit. Again, where such a receiver is provided with a separate signalchannel for AM broadcast reception, it is desirable to use one of theopposed diodes for AM detection and AVC rectication without switching.It is an important object of my invention to provide such an .FM-AMreceiver with -minimum circuit components and maximum detected voltageoutput.

In the customary limiter, or fast-acting AVC, used in FM receivers s lowvariations in carrier amplitude are controlled along with rapid varia.-tions. It is another object of my present invention to improve thereceiver performance by providing separate slow-acting AVC andfast-acting Other objects of my present invention are to improvegenerally the efficiency of IFM-AM receivers, and more especially toprovide economical detector circuits for such receivers.

Still other features will best be understood by reference to thefollowing description, taken in connection with the drawings, in which Ihave indicated diagrammatically several circuit organizations whereby myinvention may be carried into effect.

In the drawings:

Fig. l shows, in partial schematic form, an FM-AM receiver employing oneembodiment of my invention;

Fig. la shows an ideal selectivity characteristic of the FM signalchannel;

Fig. 1b shows an ideal selectivity characteristic of the AM signalchannel;

Fig. 1c illustrates the FM detection characteristic;

Fig. 1d shows the Frequency vs. AVC voltage characteristic for FMreception;

Fig. 2 is the circuit diagram of an FM signal channel using separateslow-acting and fastacting AVC and Fig. 3 shows a modication of thecircuit of Fig. 2 applied to an FM and AM receiver.

Referring now to the accompanying drawings, wherein lile referencecharacters in the dilerent figures designate similar circuit elements,Fig. 1 shows an illustrative receiving system embodying a demodulatornetwork adapted to provide audio voltage and AVC voltage in response toFM or AM signal reception. The receiver circuits prior to thedemodulator are schematically represented. Those skilled in the art ofradio reception are well acquainted with the nature of the circuitscustomarily employed in multi-band receivers. While my invention isreadily adapted for FM and AM reception on respective bands of 42 to 50megacycles (rnc.) and 550 to 1700 kilocycles (kc), it is to be clearlyunderstood that the invention is not limited to such frequency bands.The 42 to 50 mc. band Lisl presented by way of illustration, since it isthe FM broadcast band presently assigned to such transmission. The 550to 1700 kc. band is the present AM broadcast band assigned totransmission of AM signals.

It will further be understood that in the following description andclaims the generic expression angle modulated is intended to includefrequency modulation, phase modulation or hybrid modulations possessingcharacteristics common t0 either form of modulation. From a very generalviewpoint my invention relates to a demodumunication.

lator network having separate input circuits for carrier waves ofdifferent frequencies and of different modulation characteristics.

The numerals I and 2 in Fig. 1 denote respectively diferent sources ofmodulated carrier waves. Source I may be the usual signal collector,such as a dipole, employed for collecting FM waves. The FM waves aretransmitted from FM transmitters at a mean, center or carrier frequencyassigned to the particular transmitter. In the assumed FM band of 42 to50 mc. the radiated carrier wave frequency would be in that range, andwould be a wave of variable frequency and substantially uniformamplitude. As is well known, the frequency modulation of the carrierwave would be in accordance with the modulation signals at thetransmitter. The extent of frequency deviation of the carrier frequencyis a function of the modulation signal amplitude, while the rate offrequency deviation is dependent upon the modulation signal frequenciesper se. The permissible extreme frequency deviation in the FM band of 42to 50 mc. is '75 kc. to either side of the carrier frequency; theallotted FM channels are 200 kc. wide. These values are purelyillustrative.

Source 2 may be the customary grounded antenna circuit employed in AMbroadcast reception. The allotted channels are Iii kc. wide in thisband. In AM transmission the carrier Wave is modulated in amplitude inaccordance with the modulation signals. The carrier frequently ismaintained constant in value at the transmitter. The numeral 3designates a tunable radio frequency amplifier having suitable signalselector circuits for FM or AM reception. Switching devices 4 and 5respectively provide separate connection of the sources I and 2 torespective selector circuits of amplifier 3. It will be understood thatwhen switch 4 is in closed position, collected FM signal energy will beapplied to selector circuits of amplifier 3 capable of selectivelyamplifying the FM signals over a band at least 150 kc. wide. Uponclosing of switch 5 and opening switch 4 the Same amplifier 3 will havethe FM selector circuits thereof operatively replaced by AM selectorcircuits. These latter circuits will select the collected AM signals andpermit amplier 3 to amplify the same over a 10 kc. band. Multi-bandselector circuits and switching devices for suitable change-over arewell known to those skilled in the art of radio com- Switching devices 4and 5 affect the demodulator circuit only in so far as they determinethe character of the modulated wave to be delivered to the demodulator.

Assuming the system is of the superheterodyne type, as is the usualpractice at present, the converter E and intermediate frequency (I. F.)amplifier 'I will also be provided with suitable FM and AM signalselector circuits. At the converter 6 the FM signals will have the meanor center frequency thereof reduced to a value which may be chosen froma range of 1 to 2O rnc., as for example 4.3 mc. The AM signals arereduced to an I. F. of 455 kc., as an illustrative frequency value, thelatter being a commonly employed frequency in AM broadcast receivers ofthe superheterodyne type. The I. F. amplifier l, which may consist ofone or more separate stages oi amplification, will have an ultimateoutput circuit from which may be derived, at separate points thereof,the amplied FM signals or AM signals.

The selective circuits 8 and 9 are to be understood as being arranged inseries in the plate circuit of the last I. F. amplifier tube. Each ofcircuits 8 and 9 is resonated to its respective operating I. F. valuefor FM or AM reception. Thus, circuit 8 is tuned to 4.3 mc., whilecircuit 9 is tuned to 455 kc. There will be developed across tunedcircuit 8 the FM signals at the 4.3 mc. mean frequency when switch 4 isclosed, and all FM selector circuits of amplifier 3, converter 6 and I.F. amplifier 'I are in operative electrical connection. Conversely, whenswitch 5 is closed, and switch 4 is open, and all AM selector circuitsare in operative electrical connection, there will be developed acrosscircuit 9 AM signals at the I. F. value of 455 kc. The impedance ofcircuit 9 is negligible at 4.3 mc.; therefore, the insertion of circuit9 in series with circuit will not affect the development of FM signalvoltage across circuit 8. Similarly, the impedance of circuit 8 isnegligible at 455 kc., and circuit 8 will not affect development of AMsignal voltage across circuit 9.

The demodulator comprises but two electron discharge devices, shown asdiodes by way of illustration. The electrodes of the pair of diodes maybe housed within a common tube envelope, or they may be in separateenvelopes. By way of specific illustration the diodes I Il and II areshown as being separate tubes. The diode I0 is provided with a resonantinput circuit I`2 which is inductively coupled to the circuit 8. Theanode of diode I0 is connected to one side of the input circuit I2,while the cathode of diode Ill is connected to the opposite side ofinput circuit I2 through the load resistor I3. Resistor I3 is bypassedby condenser I4 for high frequency currents.

Diode I I has its cathode established at ground potential, while itsanode is connected to the high alternating potential side of itsresonant input circuit I5. Circuit I5 is also inductively coupled to thecircuit 8. The low potential side of circuit I5 is connected to groundthrough the coil I6 and load resistor I"I. Coil I6 is magneticallycoupled to circuit 9, and condenser I8 shunts coil I6 to provide aresonant circuit IE5- I3 tuned to 455 kc. Condenser I 9 shunts resistorIl to bypass high frequency currents.

The input circuits I2 and I5 of diodes Ill and II respectively areoppositely and equally mistuned with respect to the operating I. F.value for FM reception. In other words, if the FM signals developedacross circuit 8 have a center frequency of Fc (4.3 mc.), then circuitsI2 and I5 will be detuned in opposite senses by equal predeterminedfrequency values relative to Fc. It will be recognized that circuits 8,I2 and I5 provide the well known discriminator network of Conrad U. S.Patent No. 2,057,640. The action of this form of discriminator circuitis well known to those skilled in the art. It functions to translate FMhigh frequency signals into corresponding AM high frequency signals.

At the 4.3 mc. frequency used for FM reception the impedance of circuitI6-I8 is negligible, and hence the load resistor I1 is effectively inseries with input circuit I5 and diode II. lThe upper end of resistor I1is coupled to the lower end of resistor I3 through the condenser 20.Condenser 2n has a low impedance for the modulation frequenciesdeveloped during detection of the FM signals, but has a high impedanceto high frequency currents. In other words, condenser 20 is a modulationfrequency coupling condenser.

Assuming that the modulation signals on the received FM waves are ofaudio frequency, then the audio frequency amplifier of the receiverwil-l have its input lead connected to the cathode end of load; resistorI3. As already known, a deemphasis network 2l' may be employed in theaudio frequency output connection in order to compensate forpre-emphasis of higher audio frequencies at the transmitter. Inaccordance with well-understoodprinciples of FM signal detection. thealternating current components in the rectiiied signal voltage acrosseach of resistors Il and i3v will be combined in phase opposition due tothe connection of the anode end ofA resistor Il to the anode end ofresistor I3 by coupling. condenser 2D..

The diferential voltage resulting from the phasefopposed voltagescorresponds to the audio modulation signal voltage originally applied tothe. FM carrier wave at the FM transmitter.. At the same time there is.provideda conductive connection between the negative or anode end ofvresister II and the cathode or positive end of resistor I. Thisconductive connection includes resistor 22; Considered relative toground the direct current voltage components of the rectied voltagesappearing across resistors. I3 and I'I` are added in phase-aiding sense.In other words, the direct current voltage components of rectiers t andI Ir are combined in additive manner duringy FM signal reception, whilethe alternating current. (audio) outputs. ci rectiiiers Ill and. III arecombined in phase-opposed relation.

An AVC connection 2t is provided between the gain control electrodes, asfor example. the signal grids, of the various tubes in networks t, 6 and1 and the negtive endof resistor I3. The AVC connection 23 includes alter resistor 24, whose lower end isv bypassed to ground by an audiofrequency condenser 2.5, so as to prevent alternating cur.- rentcomponents. from being transmitted over the. connection 23. Network24-25. therefore acts as. a` time constant network to. produce slow AVCaction.

The functionr of the AVC connection is well known to those skilledv inthe art. Should there be.. any carrier amplitude variation at the inputterminals of each of rectifiers lil and II, such amplitude variationwill be translated into a corresponding change in direct current voltageacross the corresponding load resistors. I3 and Il. The AVC voltageapplied over connection 23- to the controlled tubes will reduce the gainof the tubes to counteract undesired carrier amplitude increase.

During AM signal reception` the I. F. signal energy produced in thecircuit 9 will be trans.- ferredto. input. circuit lli-I8. Each ofcircuits 9 and l-I 8 is tunedto the operating I. F. value of` 455 kc.The circuit I5 and diode II are both included in a series circuit withtuned circuit Iii-I8 and load resistor I'I. The circuit I5, resonantclose to 4.3 mc., has no appreciable efrect on the series circuit, sinceit acts as an eX- tremely low impedance connection at the 455 kc. value.The modulation voltage component of the rectiedl. F. energy developedacross bypassed load resistor Il is applied through condenser andresistor I3 to the common-modulation signal output circuit. The directcurrent voltage component across resistor Il is applied over AVC path 23to the prior tubes. The AVC line 23 connects to the ungrounded end of.resistor Il, through a series path consisting of resistor 24, resistorI3 and resistor 22. Here, again, the network 24--25 acts to introducetimedelay into. the AVC action.

In Fig.. lai I' have shown the form of selectivity characteristic. whichis preferred for use during FMsignal reception. The curve is idealized,and represents a flat-topped characteristic at least kc. wide. Thecharacteristic represents the idea-l pass band of the receiver circuitsup to the opposed rectiers: Ill- Ii during FM reception. The dat-toppedselectivity characteristic, if the FM carrier is correctly centered onit, insures against production of' amplitude modulation on the-FM waveas the latter passesv through the cas;- caded resonant circuits to theFM detector cire cuit, and lessons the importance of the use of anamplitude limiter stage in FM reception. The AVC circuit actseffectively to reduce the gain of the receiver tubes in response toincreases in amplitude of the FM carrier.

By Way of contrast to Fig. la I have shown the'AM selectivitycharacteristic in Fig. 1b. This curve is idealized, and represents theflat-topped l0. kc. pass band of the receiver circuits up to the rectierduring AM signal reception. This enables faithful AM reception andpermits the AVC action to function in the well understood manner. Fig.1cv shows the FM detection characteristic of opposed rectifiers I0 andII and their associated input circuits t2 and I5. It is desirable tohave the spaced peaks of the ideal curve sepa,- rated by a frequencyvalue in excess of the 150 kc. band width. Further, the curve should beas linear as possible between the peaks thereof.

With a detection characteristic as shown in Fig. 1c, the AVC voltage vs.Frequency characteristic durinng FM reception will be substantially ofthe form represented in Fig. 1d. It will be noted that the AVC (negativein polarity) voltage, with changes in carrier frequency but notinamplitude, becomes a maximum at spaced peaks of the curve with adecrease towards the center frequencyv Fc. In other words, the AVC biasWill' be a maximum on each side of Fc thereby providing an audible aidin differentiation between exact tuning of the receiver and off-centertuning thereof.

In Fig. 2 Iv have shown a receiving system of the type schematicallyrepresented in Fig. l, but arranged; to receive FM signals only. Forthis reason the circuits shown in Fig. 2 omit the AM signal receivingcircuits. The FM detector circuit is substantially the same as thatshown in Fig. 1 for receiving FM- signals. It will be seen that the AVCline 231s connected through lter resistor 2li to they anode end ofresistor I3, and therethrough to the anode end of resistor I1 throughresistor 22. The time constant circuit 24;-25 provides slow-acting AVCaction. In order to secure a. measure of delay there is employed a wellknown delay device consisting of a diode 50 whose cathode is grounded,but whose anode isV connected to the lead 23. A permanent positive biasis applied to the anode of diode 50 thereby effectively establishing thelower end of resistor 24 at ground. potential until the diode 50 isrendered non-conductive. The diode 5G becomes non-conductive upon thenegative potential atthe anode end of resistor I3 assuming a sufficientnegative value relative to ground to overcomeY the positive bias on theanode of diode 5I). From that point on the AVC bias will be suppliedVover line 23 to the control grids of the various controlled tubes. Thisform of delayed AVC action is well known.

Only the I; F. ampliers preceding the discriminatorl networkare shown,since it is tobe understoodithatvthe.. slow-acting AVCv circuit 23 maybe connected to'one or more of the transmission tubes preceding the I.F. amplifier tubes I and 52. 'I'he tuned I. F. transformers 53 and 54will, of course, be constructed so as to pass the required frequencyswing of the FM signals. It will be seen that the I. F. amplifier stagesshown in Fig. 2 are of well known form, and hence they need no furtherexplanation. The input electrode of amplifier 52 is magnetically coupledto the resonant secondary circuit of I. F. transformer 54, and,therefore, derives its signal energy therefrom.

A diode rectifier 55 has its electrodes coupled to the opposite sides ofthe secondary circuit of I. F. transformer 54. The resistor 56. bypassedby condenser 5l for I. F. currents, is connected between the groundedcathode of diode 55 and the low potential side of the secondary circuitof transformer Eli. Alternating current components of the rectifiedvoltage developed across resistor 56 are applied to the control grids ofamplifiers 5I and 52 by connecting the grid circuits of these tubes toany desired point on resistor 56 by means of an audio coupling condenser58. The feedback to the amplifier control grids from across resistor 56is performed degeneratively so as to compensate for undesired relativelyfast amplitude variations of the FM signals. This action will bereferred to hereinafter as control of fast variations or fast AVC. Itwill be noted that the slow-acting AVC lead 23 is also connected to thegrids of tubes 5I and 52 through a filter resistor 59.

No amplitude limiter stage per se is employed prior to the discriminatornetwork. Furthermore, it will be observed that there is utlized separatecontrol by diodes 55 and Ill-II respectively of fast variations and slowvariations in carrier amplitude. In the familiar and customary form oflimiter, or fast-acting AVC, slow variations in carrier amplitude arecontrolled along with the rapid variations. The customary limitercircuit of necessity must reduce the -average amplification of thereceiver in order to be able to increase and decrease the overall gainto compensate for rapid variations of amplitude. By using a separateslow-acting delayed AVC circuit, together with fast AVC, the averageamplification of the receiver will not be cut for weak signals. However,variations in the carrier amplitude, for example as the carrier is swungacross a round-top selectivity curve, will be compensated for by theamplitude correction device. Hence, weak signals will be received withless distortion.

Since the gain of a special limiter stage is usually low, because it isusually operated with low screen and plate voltages, there is lanotheradvantage secured by the circuit of Fig. 2. By using the separatecontrols high gain amplification is obtained. It will be noted that theslow-acting AVC voltage is derived from the discriminatordetector outputcircuit as explained in connection with Fig. l. This AVC voltage isconsiderably larger than can be secured in the case where a limite-rstage precedes the discriminator thereby giving better regulation andhaving less likelihood of overloading preceding stages.

The fast-acting AVC rectifier applies its rectified voltage to pointsprior to, and following, the point from which it derives its signalenergy. This insures a substantially fiat gain control action. Theslow-acting AVC voltage is applied to many tubes in addition to thetubes to which the fastacting AVC bias is applied. By virtue of thecombined AVC actions of Fig. 2 the response char- 8 acteristics of thecoupling transformers between stages need not be ideally flat, since theAVC circuit will tend to compensate for any curvature in the responsecharacteristics.

In Fig. 3 I have shown a further modification wherein the fast-actingAVC voltage may be secured from the discriminator network. This has theadvantage that the control voltage obtainable is much greater, and is,therefore, more effective. It will be recognized that in Fig. 3 theFM-AM networks feeding the diodes I and I I are very similar to thoseshown in Fig. 1. Assuming first that FM signals are being received, theload resistors I3 and I'I develop thereacross audio voltages which arecombined in phase opposition and the resultant voltage is transmittedthrough resistor 2| to the subsequent audio amplifier. The directcurrent voltage components across resistors I3 and I'I are combined inadditive phase through resistor 22. The slow-acting AVC connection 23goes through resistor 24 to the negative end of resistor I3. The AVCaction is delayed by connecting the lead 23 to a source of positivebias. Instead of using the diode 50 of Fig. 2 for the delaying action,however, the resistor 59 of Fig. 2 may be omitted and the control gridsof the various tubes to be controlled may be permitted to draw gridcurrent thereby rendering the AVC circuit ineffective until the negativevoltage at the anode end of resistor I3 becomes sufficiently negative toovercome the positive bias. This is, also, a well-known means forsecuring delayed AVC action.

The fast-acting AVC action is secured by means of an `auxiliary dioderectifier E0 whose cathode is connected to the same side of circuit I2as the anode of diode I0. The anode of diode 6U is connected to theopposite side of circuit I2 through a resistor SI bypassed by condenserG2. The resistor 6I functions as the load resistor for diode 60, and thecondenser 62 bypasses only the I. F. currents. The fast-acting AVCconnection 63 utilizes the alternating current voltage components of therectified voltages across resistor 5I and resistor I'I. Since thepositive end of resistor SI is connected through the audio couplingcondenser 20 to the negative end of resistor I1, it will be seen thatthe audio voltages across resistors Il and BI are combined in additivephase for transmission through the audio coupling condenser 'ID toconnection 63 when switch 'II is closed. Switch II is provided toselectively connect or disconnect connection 63 from the condenser 10.Condenser 'I0 and resistor l2, the latter having its lower end grounded,remove the direct current components from the voltage and provide thefast time constant network for connection 63.

It is contemplated that during FM signal reception the AVC connect-ion63 will be made to at least the prior I. F. amplifier l, and will actdegeneratively as described in reference to Fig. 2. The slow-acting AVCconnection 23 will also be made to one or more of the transmission tubespreceding stage l. For AM reception the switch 'II may be opened therebyremoving the fastacting AVC circuit. In this case the AVC lead 23 isconnected through resistor 2li, resistor I3 and resistor 22 to thenegative end of resistor II. In other words the direct current voltagecomponent across resistor I l is still used for the slow AVC action. InAM reception the audio voltage component across resistor I'I is appliedto the audio output connection through the path cornprising condenser 29and resistor I3. It will, therefore, be appreciated that in Fig. 3 Ihave provided a circuit wherein both slow-acting AVC and fast-acting AVCderive their voltages from the combination of rectified voltages inadditive phase and in response to FM signal reception. For AM receptiononly one of the three rectifiers need be employed with its associatedload resistor. In this Way distortionless AM reception can be securedwith only the slow-acting AVC circuit operating.

While I have indicated and described several systems for carrying myinvention into effect, it will he apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention.

What I claim is:

1. In a frequency modulation receiver, a frequency discriminatorincluding a pair of load resistors across which direct current voltagesappear as a result of detection, conductive means for connecting saidresistors in series-aiding relation with respect to said voltages, aslow-acting circuit for applying the resultant of the voltages to aportion of the receiver as a first gain control voltage thereof, meansfor deriving from the received frequency modulated signal a secondcontrol voltage which is representative of relatively fast amplitudevariations of the signal, means coupled to said resistors for deriving adetected modulation voltage representative of the frequency modulationof the received signal, and a fast-acting circuit for applying saidsecond control voltage to a portion of the receiver as a second gaincontrol voltage.

2. In a yfrequency modulation receiver, a frequency discriminatorincluding a pair of load resistors across which direct current voltagesappear as a result of detection, a conductive impedance element forconnecting said resistors in series-aiding relation with respect to saidvoltages, a slow-acting circuit for applying the resultant of thevoltages to a portion of the receiver as a gain control voltage thereof,means I:for deriving from the received frequency modulated signal amodulation voltage which is representative of relatively fastamplitudevariations of the signal, and a fast-acting circuit for applying saidmodulation voltage to a portion of the receiver as a gain controlvoltage, means for connecting said resistors in series-opposing relationwith respect to the modulation voltages thereacross representative ofdesired frequency modulation thereof, and means for utilizing theresultant of the series-opposed modulation voltages.

3. In a frequency modulation receiver provided with an amplifier, afrequency discriminator coupled to the amplier providing a pair ofsignal voltages whose relative magnitudes are dependent on the frequencyVariation of received waves, a pair of rectiflers each having an inputelectrode separately coupled to said discriminator to have a respectiveone of the pair of signal voltages applied thereto, a separate loadimpedance in circuit with each rectifier, a circuit connected acrosssaid load impedances deriving a modulation voltage in response to theadded rectied voltages developed across said load impedances, aconductive impedance element connecting opposite polarity ends of saidload impedances in series relation, a iirst gain control connection tosimilar polarity ends of said impedances for deriving from saidimpedances in additive polarity sense slow-acting rectified voltagesdeveloped thereacross, means to apply the additive voltage to saidamplifier, an auxiliary rectifier having an input electrode coupled tosaid discriminator and including a third load impedance in circuittherewith, means of low impedance to modulation currents connectingopposite polarity ends of one of said first two impedances and saidthird impedance, a second gain control connection between said ampliflerand one end of the third impedance for deriving lfrom said thirdimpedance and said one impedance in additive polarity sense thefast-acting rectified modulation voltages thereacross and for impressingthem on said amplifier.

4. In a frequency modulation receiver, a frequency discriminatorincluding a pair of load resistors across which direct current voltagesappear as a result of frequency discrimination, a conductive impedanceelement for connecting said resistors in series-aiding relation withrespect to said voltages, a slow-acting circuit for applying theresultant of the voltages to a portion of the receiver 'as a first gaincontrol voltage thereof, means for deriving from the received frequencymodulated signal a second control voltage which is representative ofrelatively f-ast amplitude variations of the signal, a circuit connectedto said -resistors for deriving a detected modulation voltagerepresentative of desired frequency modulation of the received signal,said means for deriving comprising a rectifier in circuit with saiddiscriminator, a third load resistor in circuit with the rectifier,means connecting one of the two load resistors and one terminal of thethird resistor in series-aiding relation with respect to said secondcontrol voltage, and a fastacting circuit connected to the otherterminal of the third resistor.

WINFIELD R. KOCH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS- Number Name Date 2,057,640 Conrad Oct. 13, 19362,264,724 Schonfeld Dec. 2, 1941 2,330,902 McCoy Oct. 5, 1943

